STRUCTURE OF PROTOZOA. 99 



of many Protozoa, as in the cells of higher animals, the 

 nucleus plays an important part. It passes out of a resting 

 state and becomes active. The nuclear threads or " chrom- 

 atin filaments " loosen themselves from their coiled state, 

 and arrange themselves in a star at the equator of the cell, 

 whence they divide into two groups, which retreat from one 

 another, and become the daughter nuclei of two daughter 

 cells. 



In naked Protozoa, the outer part of the cell-substance 

 (" ectoplasm ") is often different from the inner part (" endo- 

 plasm "), as one would expect it to be, but this difference is 

 a physical one of little importance. In the other Protozoa, 

 there is a more definite rind or thickened margin of cell- 

 substance. Outside this there may be a " cuticle " distinct 

 from the living matter, sometimes consisting of chitin, or 

 gelatin, or in a few of cellulose. The cuticle may form 

 a cyst, which is either a protection during drought or a 

 sheath within which the unit proceeds to divide into numerous 

 spores. Moreover, the cuticle may become the basis of a 

 shell, formed from foreign particles, or made by the animal 

 itself of lime, flint, or " horny " material. 



In the cell-substance, there may be bubbles of water 

 taken in with food-particles (food vacuoles), special pulsating 

 regions which sometimes burst to the exterior like primitive 

 excretory appliances (contractile vacuoles), fibres which seem 

 to be specially contractile (in Gregarines), spicules of flint or 

 threads of horn-like material which may build up a connected 

 framework, pigment of various kinds perhaps including 

 chlorophyll. In the Radiolarians and a few Foraminifera, 

 there are partner plant cells or symbiotic Algae. 



Reproduction of Protozoa. — Growth and reproduction are 

 on a different plane from the other functions. Growth 

 occurs when income exceeds expenditure, when constructive 

 or anabolic processes are in the ascendant. Reproduction 

 occurs at the limit of growth, or sometimes in disadvan- 

 tageous conditions when disruptive or katabolic processes 

 gain some relative predominance. 



As it ds by cell-division that all embryos are formed 

 from the egg, and all growth is effected, the beginnings of this 

 process are of much interest, (a) Some very simple Protozoa 

 seem to reproduce by what looks like the rupture of outlying 



